Siglecs (sialic acid-binding immunoglobulin-like lectins) are a recently designated family of cell surface molecules that are a subset of the immunoglobulin (Ig) gene superfamily (Crocker, P. R., and Varki, A. (2001) Immunology 103, 137-145; Crocker, P. R., and Varki, A. (2001) Trends Immunol. 22, 337-342; Crocker, P. R. (2002) Curr Opin Struct Biol 12, 609-615). Siglecs differ from traditional Ig superfamily members in several ways. While their extracellular domains contain a variable number of C2-set Ig domains, unlike other Ig superfamily members, Siglecs possess an N-terminal V-set Ig domain that binds sialylated structures (May, A. P., Robinson, R. C., Vinson, M., Crocker, P. R., and Jones, E. Y. (1998) Mol Cell 1, 719-728; Yamaji, T., Teranishi, T., Alphey, M. S., Crocker, P. R., and Hashimoto, Y. (2002) J Biol Chem 277, 6324-6332; Zaccai, N. R., Maenaka, K., Maenaka, T., Crocker, P. R., Brossmer, R., Kelm, S., and Jones, E. Y. (2003) Structure (Camb) 11, 557-567; Alphey, M. S., Attrill, H., Crocker, P. R., and van Aalten, D. M. (2003) J Biol Chem 278, 3372-3377; Dimasi, N., Moretta, A., Moretta, L., Biassoni, R., and Mariuzza, R. A. (2004) Acta Crystallogr D Biol Crystallogr 60, 401-403). In addition, Siglec cytoplasmic domains typically contain multiple tyrosine residues, including some with consensus immunoreceptor tyrosine-based inhibitory motifs (ITIMs). This suggests that Siglecs possess signal transduction activity. Direct evidence of signaling has already been shown for several human Siglecs (Falco, M., Biassoni, R., Bottino, C., Vitale, M., Sivori, S., Augugliaro, R., Moretta, L., and Moretta, A. (1999) J. Exp. Med. 190, 793-802; Mingari, M. C., Vitale, C., Romagnani, C., Falco, M., and Moretta, L. (2001) Immunol. Rev. 181, 260-268; Whitney, G., Wang, S., Chang, H., Cheng, K. Y., Lu, P., Zhou, X. D., Yang, W. P., McKinnon, M., and Longphre, M. (2001) Eur. J. Biochem. 268, 6083-6096; Nicoll, G., Avril, T., Lock, K., Furukawa, K., Bovin, N., and Crocker, P. R. (2003) Eur J Immunol 33, 1642-1648; Ikehara, Y., Ikehara, S. K., and Paulson, J. C. (2004) J Biol Chem).
Siglec-8 (alternate name: SAF-2, sialoadhesin family-2) was discovered by CD33 homology screening of expressed sequence tag sequences from a cDNA library made from a human eosinophil cDNA library (Kikly, K. K., Bochner, B. S., Freeman, S., Tan, K. B., Gallagher, K. T., D'Alessio, K., Holmes, S. D., Abrahamson, J., Hopson, C. B., Fischer, E. I., Erickson-Miller, C. L., Tachimoto, H., Schleimer, R. P., and White, J. R. (2000) J. Allergy Clin. Immunol. 105, 1093-1100; Floyd, H., Ni, J., Cornish, A. L., Zeng, Z., Liu, D., Carter, K. C., Steel, J., and Crocker, P. R. (2000) J. Biol. Chem. 275, 861-866). Highest levels of homology were found between Siglec-8 and Siglec-3 (49%), Siglec-5 (42%) and Siglec-7 (68%), with virtually all of the homology due to similarities in the extracellular and transmembrane regions. Subsequently, a splice variant of Siglec-8, termed Siglec-8L, containing an identical extracellular domain but a longer cytoplasmic tail possessing two tyrosine-based motifs, was discovered from human genomic DNA (Foussias, G., Yousef, G. M., and Diamandis, E. P. (2000) Biochem Biophys Res Commun 278, 775-781; Yousef, G. M., Ordon, M. H., Foussias, G., and Diamandis, E. P. (2002) Gene 286, 259-270). Additional experiments, using monoclonal antibodies, revealed that Siglec-8 was not only expressed on the surface of eosinophils, but on basophils and mast cells as well (Kikly, K. K., Bochner, B. S., Freeman, S., Tan, K. B., Gallagher, K. T., D'Alessio, K., Holmes, S. D., Abrahamson, J., Hopson, C. B., Fischer, E. I., Erickson-Miller, C. L., Tachimoto, H., Schleimer, R. P., and White, J. R. (2000) J. Allergy Clin. Immunol. 105, 1093-1100) and the existence of both the Siglec-8 and Siglec-8L isoforms was verified in human eosinophils, basophils and mast cells (Aizawa, H., Plitt, J., and Bochner, B. S. (2002) J. Allergy Clin. Immunol. 109, 176; Nutku, E., Aizawa, H., Tachimoto, H., Hudson, S. A., and Bochner, B. S. (2004) in Allergy Frontiers and Futures, Proceedings of the 24th Symposium of the Collegium Internationale Allergologicum (Bienenstock, J., Ring, J., and Togias, A. G., eds), pp. 130-132, Hogrefe and Huber, Cambridge Mass.). Most recently it was demonstrated that antibody crosslinking of Siglec-8 on human eosinophils induced caspase-dependent apoptosis in vitro (Nutku, E., Aizawa, H., Hudson, S. A., and Bochner, B. S. (2003) Blood 101, 5014-5020).
The search for Siglec ligands remains rather complex. Many of the Siglecs recognize α2-3- and α2-6-linked sialic acids (Freeman, S. D., Kelm, S., Barber, E. K., and Crocker, P. R. (1995) Blood 85, 2005-2012; Brinkman-Van der Linden, E. C., and Varki, A. (2000) J. Biol. Chem. 275, 8625-8632) while others bind to other sialylated structures. For example, Siglec-1 has been shown to bind the highly glycosylated surface protein CD43 (van den Berg, T. K., Nath, D., Ziltener, H. J., Vestweber, D., Fukuda, M., van Die, I., and Crocker, P. R. (2001) J. Immunol. 166, 3637-3640), the epithelial mucin MUC-1 (Nath, D., Hartnell, A., Happerfield, L., Miles, D. W., Burchell, J., Taylor-Papadimitriou, J., and Crocker, P. R. (1999) Immunology 98, 213-219) and sialylated lipopolysaccharide (Jones, C., Virji, M., and Crocker, P. R. (2003) Mol Microbiol 49, 1213-1225). Among a panel of glycans tested, Siglec-3 showed enhanced binding to a multivalent form of sialyl-Tn (NeuAcα2-6GalNAc) disaccharides (Brinkman-Van der Linden, E. C., Angata, T., Reynolds, S. A., Powell, L. D., Hedrick, S. M., and Varki, A. (2003) Mol Cell Biol 23, 4199-4206). Siglec-7 binds to GD3, LSTb, sialyl Lewisa and NeuAcα2-8NeuAc, while Siglec-9 preferentially binds GD1a and LSTc (Rapoport, E., Mikhalyov, I., Zhang, J., Crocker, P., and Bovin, N. (2003) Bioorg Med Chem Lett 13, 675-678; Miyazaki, K., Ohmori, K., Izawa, M., Koike, T., Kumamoto, K., Furukawa, K., Ando, T., Kiso, M., Yamaji, T., Hashimoto, Y., Suzuki, A., Yoshida, A., Takeuchi, M., and Kannagi, R. (2004) Cancer Res 64, 4498-4505). For Siglec-8, it has been shown that red blood cell rosettes are formed with Siglec-8, and neuramimidase treatment alters rosette formation (Kikly, K. K., Bochner, B. S., Freeman, S., Tan, K. B., Gallagher, K. T., D'Alessio, K., Holmes, S. D., Abrahamson, J., Hopson, C. B., Fischer, E. I., Erickson-Miller, C. L., Tachimoto, H., Schleimer, R. P., and White, J. R. (2000) J. Allergy Clin. Immunol. 105, 1093-1100; Floyd, H., Ni, J., Cornish, A. L., Zeng, Z., Liu, D., Carter, K. C., Steel, J., and Crocker, P. R. (2000) J. Biol. Chem. 275, 861-866). Specific structures shown to bind Siglec-8 include forms of sialic acid linked α2-3 or α2-6 to Galβ1-4GlcNAc (Floyd, H., Ni, J., Cornish, A. L., Zeng, Z., Liu, D., Carter, K. C., Steel, J., and Crocker, P. R. (2000) J. Biol. Chem. 275, 861-866). In a more comprehensive evaluation of binding specificities, ten Siglec-Ig chimeras were screened for binding to 28 different sialoside-streptavidin-alkaline phosphatase probes, and a wide range of binding patterns were observed, but there was no clear binding preference for Siglec-8 (Blixt, O., Collins, B. E., van den Nieuwenhof, I. M., Crocker, P. R., and Paulson, J. C. (2003) J Biol Chem 278, 31007-31019).
Pharmacological activation of Siglec-8 would be expected to deplete eosinophils, basophils, and mast cells from the body and/or reduces their activation level. This kind of pharmacologic effect may be used for treatment of diseases including asthma, allergic diseases, atopic dermatitis, hypereosinophilic syndromes, mastocytosis, leukemias, sinusitis, nasal polyposis, urticaria and anaphylaxis.
Given the prevalence of asthma and other allergic conditions, there remains a need for effective prophylactic and therapeutic treatment of these disorders, in particular, those related disorders associated with Siglec-8 expressing cells in a subject.